System and method for determining patient posture based on 3-D trajectory using an implantable medical device

Abstract

Time-varying spatial signals are detected by accelerometers mounted within the patient. The signals, representative of the actual 3-D trajectory of the patient, are compared with information representative of expected trajectories retrieved from memory to identify a current patient posture, which may be either a dynamic posture such as walking or running or a change in posture such as rising from a seated position to a standing position. In this manner, a change in posture of the patient is identified based upon a full 3-D trajectory, rather than merely the orientation of the patient at the beginning and the end of the change in posture. In an example described herein, the implantable device stores information representative of expected 3-D trajectories in the form of pre-calculated comparison matrices derived from orthonormal kernels employing Laguerre functions or Lagrange functions. A technique is also described for use by an external programmer for pre-calculating comparison matrices so as to reduce the processing burden within the implanted device during posture detection.

1. For use in an implantable medical device for implant within a patient, a method comprising:
 storing one or more trajectory templates in memory; 
 detecting time-varying signals representative of a trajectory of the patient;
 generating a trajectory based on the time-varying signals; and 
 identifying patient posture based on a comparison of the trajectory with the one or more trajectory templates. 
 
 
   
   
     2. The method of  claim 1  wherein identifying patient posture comprises:
 retrieving information representative of trajectories associated with selected postures of interest; and 
 identifying patient posture from the time-varying signals representative of the trajectory of the patient in combination with the information representative of trajectories associated with the postures of interest. 
 
   
   
     3. The method of  claim 1  wherein the time-varying signals are time-varying 3-D acceleration signals. 
   
   
     4. The method of  claim 3  wherein identifying patient posture comprises:
 integrating the time-varying 3-D acceleration signals to derive time-varying 3-D position signals; and 
 identifying patient posture based on the time-varying 3-D position signals. 
 
   
   
     5. The method of  claim 1  wherein identifying patient posture based on the time-varying signals further comprises identifying a change in posture. 
   
   
     6. The method of  claim 1  wherein identifying patient posture based on the time-varying signals comprises:
 retrieving information representative of 3-D spatial trajectories associated with a posture of interest in the form of a comparison matrix Z wherein Z=(H T *H) −1 *H T  where H is a matrix of orthonormal kernels (K 1 , K 2 , K 3 ) each representing the expected motion of the patient along an orthonormal axis; 
 integrating the time-varying signals representative of the trajectory of the patient to yield a matrix P of orthonormal vectors (P 1 , P 2 , P 3 ) each representing the actual motion of the patient along a respective orthonormal axis; and 
 determining whether the actual patient posture corresponds to the posture of interest by determining whether P sufficiently matches H by evaluating a correction vector C wherein C=Z*P. 
 
   
   
     7. The method of  claim 6  wherein determining whether P sufficiently matches H comprises:
 calculating a distance D of the correction vector C from a vector I wherein I=[1,1,1]; and 
 determining whether the distance D is less than a predetermined threshold distance. 
 
   
   
     8. The method of  claim 6  wherein determining whether P sufficiently matches H comprises:
 calculating a distance D of the correction vector C from a pre-stored vector A, wherein A represents a pre-calculated average of a corresponding vector C for the particular patient in which the stimulation device is implanted; and 
 determining whether the distance D is less than a predetermined threshold distance. 
 
   
   
     9. The method of  claim 1  further comprising:
 controlling operation of the implantable medical device based on the identified posture. 
 
   
   
     10. The method of  claim 9  wherein the implantable medical device comprises an implantable cardiac stimulation device operative to deliver pacing pulses for delivery to the heart of the patient and wherein controlling operation of the implantable medical device based on the identified posture comprises selectively adjusting a pacing rate based in patient posture. 
   
   
     11. The method of  claim 9  wherein the implantable medical device comprises an implantable cardiac stimulation device operative to deliver shocking pulses for delivery to the heart of the patient and wherein controlling operation of the implantable medical device based on the identified posture comprises selectively inhibiting shocking pulses based on patient posture. 
   
   
     12. The method of  claim 11  wherein selectively inhibiting shocking pulses based on patient posture comprises inhibiting delivery of shocking pulses while the patient is in a posture associated with walking up stairs, walking down stairs, running or pedaling. 
   
   
     13. In an implantable medical device for implant within a patient, a system comprising:
 a memory operative to store information representative of trajectories associated with a posture of interest; 
 an accelerometer operative to detect time-varying signals representative of an actual trajectory of the patient; and 
 a trajectory-based posture determination system operative to identify patient posture by generating a trajectory from a plurality of temporally-spaced ones of the time-varying signals and by comparing the trajectory with the information representative of trajectories associated with the posture of interest. 
 
   
   
     14. The system of  claim 13 
 wherein the memory stores information representative of 3-D spatial trajectories associated with a particular posture of interest in the form of a comparison matrix Z wherein Z=(H T *H) −1 *H T  where H is a matrix of orthonormal kernels (K 1 , K 2 , K 3 ) each representing the motion of the accelerometer along an orthonormal axis expected to be associated with the particular posture of interest; 
 wherein the posture determination system integrates the time-varying spatial signals representative of the actual 3-D trajectory of the patient to yield a matrix P of orthonormal vectors (P 1 , P 2 , P 3 ) each representing the actual motion of the accelerometer along a respective orthonormal axis; and 
 wherein the posture determination system determines whether the actual patient posture corresponds to the particular posture of interest by determining whether P sufficiently matches H by evaluating a correction vector C wherein C=Z*P. 
 
   
   
     15. The system of  claim 14  wherein the posture determination system determines whether P sufficiently matches H by calculating a distance D of the correction vector C from a vector I wherein I=[1,1,1] and then determining whether the distance D is less than a predetermined threshold distance. 
   
   
     16. The system of  claim 14  wherein the posture determination system determines whether P sufficiently matches H by calculating a distance D of the correction vector C from a pre-stored vector A, wherein A represents a pre-calculated average of a corresponding vector C for the particular patient in which the stimulation device is implanted, and then determining whether the distance D is less than a predetermined threshold distance. 
   
   
     17. The system of  claim 14  wherein the idealized orthonormal kernels (K 1 , K 2 , K 3 ) are represented by one or more of Laguerre functions and Lagrange functions. 
   
   
     18. The system of  claim 13  wherein the posture determination system is further operative to control operation of the implantable medical device based on the identified posture. 
   
   
     19. The system of  claim 18  wherein the implantable medical device comprises an implantable cardiac stimulation device operative to deliver pacing pulses for delivery to the heart of the patient and wherein the posture determination system selectively adjusts a pacing rate based in patient posture. 
   
   
     20. The system of  claim 13  wherein the implantable medical device comprises an implantable cardiac stimulation device operative to deliver shocking pulses for delivery to the heart of the patient and wherein the posture determination system selectively disables shocking pulses based on patient posture. 
   
   
     21. The system of  claim 20  wherein the posture determination system disables delivery of shocking pulses while the patient is in a posture associated with walking up stairs, walking down stairs, running or pedaling. 
   
   
     22. A system for use with an implantable medical device for implant within a patient, the system comprising:
 means for storing information representative of 3-D trajectories associated with a posture of interest; 
 means for detecting time-varying signals representative of an actual 3-D trajectory of the patient; 
 means for generating a trajectory based on the time-varying signals; and 
 means for identifying patient posture by comparing the trajectory with 3-D trajectories associated with the posture of interest.